Imagine you are exploring a stream bank and notice that the stream has carved into the bedrock, revealing several rock layers. You might want to figure out how old the rock layers are.

One way scientists do this is by using index fossils. Index fossils are those that are widespread and are found only in rocks from very specific time periods. Along with other methods like radiometric dating, index fossils help scientists find and mark layers of rock and the corresponding ages of geologic time. For example, a particular type of extinct clam (from the genus Venericardia) is an index fossil for the Eocene epoch. If you look back at the rock strata in the creek bed and notice that one layer contains fossils of this clam, you know that rock layer dates to the Eocene and is between 33.9 and 56 million years old. From there, you can start to figure out the ages of the rock layers above and below it.

Paleontologist Bill Berry used the fossil collections at the University of California Museum of Paleontology to identify some new index fossils. Berry studied graptolites, filter feeders that likely lived together in colonies in the ancient oceans, similar to their living relatives, the pterobranch hemichordates. Graptolites preserve in the fossil record as long, thin marks that resemble pencil scratchings. These were actually parts of the animal’s colonial housing structure, the tubarium. Although they might not look much like us, graptolites are distant cousins of today’s vertebrates – like humans.

Berry was interested in a period of geologic time around 430 to 416 million years ago, the Silurian Period, when graptolites were thriving in Earth’s oceans. He examined the shape and size of different graptolite species, compared the species between different layers of rock, and discovered that some graptolite species are only found in particular layers. Berry realized these graptolite species were perfect index fossils and used this information to label new sub-units of geologic time within the Silurian.

No one was around 420 million years ago to observe our graptolite cousins in the ancient seas, but scientists today can use their fossils to identify when particular rock layers were laid down and figure out the timing of evolutionary and ecological events long past. Berry’s discovery changed the way paleontologists think about the Silurian Period forever, thanks to his detailed research and a museum collection full of fossils.